1

Unraveling the Multiple Effects Originating the

Increased Oxidative Photoactivity of {001}-Facet

Enriched Anatase TiO2

Michela Maisano,† Maria Vittoria Dozzi,*

,† Mauro Coduri,

‡ Luca Artiglia,

§ Gaetano

Granozzi,§ and Elena Selli*

,†

† Dipartimento di Chimica, Università degli Studi di Milano, via Golgi 19, I-20133 Milano,

Italy

‡ CNR-IENI Institute for Energetics and Interphases, Corso Promessi Sposi 29, I-23900

Lecco, Italy

§ Dipartimento di Scienze Chimiche, Università degli Studi di Padova, via Marzolo 1, I-35131

Padova, Italy

2

ABSTRACT: Crystal shape control on a series of anatase photocatalysts was achieved by

varying the amount of HF employed as a capping agent in their hydrothermal synthesis. A

systematic comparison between their physico-chemical properties, determined by several

complementary surface and bulk techniques, before and after thermal treatment at 500 °C,

allowed one to discern the influence of the relative amount of exposed {001} crystal facets

among a series of effects simultaneously affecting their oxidative photocatalytic activity. The

results of both formic acid and terephthalic acid photooxidation test reactions point to the

primary role played by calcination in making {001} facets effectively photoactive. Annealing

not only removes most of the residual fluorine capping agent from the photocatalyst surface,

thus favoring substrate adsorption, but also produces morphological modifications to a crystal

packing which makes accessible a larger portion of surface {001} facets, due to unpiling of

plate-like crystals. The photocatalyst bearing the highest amount of exposed {001} facets

(60%) shows the highest photoactivity in both the direct and the •OH radical mediated

photocatalytic test reaction.

KEYWORDS: photocatalysis, anatase {001} facets; surface fluorination; photocatalytic

oxidation, holes, •OH radicals

3

1. INTRODUCTION

The attractive properties of titanium dioxide as photocatalyst have been thoroughly

investigated in the last decades.1 Among the different features affecting photoactivity, surface

properties obviously exert a dominant influence,2 reactant adsorption and electron transfer at

the interface between the solid and the reactant-containing medium being fundamental steps

in photocatalytic processes.3 Moreover, the photoactivity of the solid surface vary depending

on the exposed crystal facets.4

As to anatase, regarded as the most photocatalytically active titania polymorph,5,6

the {101}

and {001} facets differ in surface energy, number of undercoordinated Ti atoms and position

of the conduction and/or valence band edge.7,8

Since Yang et al.9 first succeeded in increasing

the abundance of high-energy {001} facets by changing their relative stability with respect to

{101} ones, several attempts have been made of tailoring the anatase crystal shape10

and

understanding how the photocatalytic activity is affected by the surface composition in terms

of exposed facets. The performance of shape-controlled anatase has been tested in a number

of photocatalytic reactions of environmental interest, e.g. Cr(VI) reduction,11–13

ammonia

oxidation14

and the degradation of several organic pollutants.15,16

Yet, no general conclusion

has been drawn about either the photoactivity order of anatase crystal facets or their optimal

balance, since different reports provided contradictory results.17–21

This is partly due to the

great variety of employed test reactions, involving different substrates adsorbed onto the

photocatalyst from the liquid or the gas phase and undergoing oxidative or reductive

transformations. Furthermore, the specific role of crystal facets needs to be clearly

distinguished from other simultaneous contributions to photoactivity, related to the adopted

shape control strategy. This is the case of capping agents, commonly employed to

preferentially stabilize the {001} facets during the crystal growth,7,22,23

that are difficult to be

4

completely removed from the photocatalyst surface24,25

and may influence the adsorption of

reactants to an extent depending on their residual amount.

The present work is focused on evaluating the efficiency towards organics photooxidation

of a series of anatase samples prepared via a hydrothermal route in the presence of different

amounts of HF, added as a capping agent to modulate the ratio of {001} to {101} facets. By

systematically comparing the intrinsic structural features of the prepared materials with their

photoactivity before and after calcination, performed to remove residual fluorine from the

anatase surface,18,19,26

we were able to highlight the role of the amount of exposed {001}

facets among a series of concurrent effects related to specific surface area, surface

fluorination and organization of anatase crystals into aggregates.

In choosing the photocatalytic test reactions we tried to avoid those in which specific

factors, other than the intrinsic performance of the photocatalyst, influence the output of the

experiment. This occurs, for example, when organic dyes are chosen as test substrates,27–30

because they may undergo self-sensitized degradation or be photocatalytically decomposed

through either oxidative or reductive paths. Therefore we selected a test reaction based on a

transparent substrate, which provides evidence of the inherent oxidative ability of

photocatalytic materials, i.e. formic acid photomineralization in aqueous suspension.31

Indeed,

this model organic substrate is converted into CO2 through its direct oxidation by

photogenerated valence band holes,32

with no stable intermediates formation, thus allowing a

straightforward assessment of photoactivity.33

The synthesized anatase photocatalysts were

also tested in the oxidation of another transparent substrate, i.e. in terephthalic acid

conversion into 2-hydroxyterephthalic acid,34–36

which is largely employed as a selective test

of •OH radical-mediated photocatalytic oxidation.

5

2. EXPERIMENTAL SECTION

2.1. Photocatalysts preparation. A series of photocatalysts, denoted as HT series, was

synthesized through a hydrothermal route.37

A fixed amount of titanium isopropoxide (5 mL)

was mixed under stirring for 15 min in a Teflon liner with different volumes of a 48 wt.% HF

solution and different amounts of water (see Table S1 in the Supporting Information) up to a

5.6 mL final volume. Only in the case of the highest content of fluorine the final volume was

larger, i.e. 6.2 mL. A reference sample was prepared under the same conditions by simply

adding 0.6 mL of water (and no HF) to titanium isopropoxide. The liner was then transferred

into a stainless-steel autoclave, which was closed and heated at 180 °C in an electric oven for

24 h. Once cooled down naturally to room temperature, the precipitate was washed several

times with water, until the fluoride ion concentration, determined by ion chromatography with

conductivity detection (Metrohm 761 Compact IC), was below 5 ppm. The precipitate was

then collected, dried at 70 °C overnight and ground into a fine powder in an agate mortar. The

so-obtained materials were labeled as HT_X, where HT refers to the employed hydrothermal

preparation method, while X stands for the nominal F/Ti molar ratio, ranging from 0 to 2.0.

In order to optimize the method for surface fluorine removal, the HT_1 sample was either

calcined at different temperatures, i.e. 400, 500 and 600 °C, or washed with a 0.1 M NaOH

solution. As detailed in the Supporting Information, the best photoactivity result in formic

acid photodegradation was attained after annealing at 500 °C (see Table S2). The relatively

poorer result obtained after washing HT_1 with NaOH is probably due to the presence of

residual surface sodium (4.3 at.%) on TiO2, detected by XPS measurements.38

Therefore, a

portion of all samples of the HT series was calcined in air at 500 °C for 2 h to remove surface

fluorine39

and the so obtained photocatalysts were labeled HT_500_X.

All reagents were purchased from Aldrich and employed as received. Water purified by a

Milli-Q water system (Millipore) was used throughout.

6

2.2. Characterization. X-ray powder diffraction (XRPD) patterns were collected on a

Panalytical X'Pert Pro diffractometer, using Ni-filtered Cu K radiation, at a scan rate of 0.05

degree s-1

. Structural and line profile analysis was performed by means of the GSAS

software40

by fitting XRD lines using a modified Pseudo-Voigt function.41

Instead of using

the Scherrer formula, often employed to determine the particle size by the fit of a single

Bragg reflection,12,20,42,43

the line profile analysis from Rietveld refinements was employed,

which takes into account all the reflections of the diffraction pattern, thus allowing the

contribution of the particle size to be discerned from that of the lattice strain.44

In this framework, crystal shape anisotropy is described by means of two parameters (p1

and p2) accounting for hkl-dependent reflection broadening, from which average values of the

crystal width (w) and thickness (t) were calculated, as follows:45

λ=

Kw

p

1

(1)

λ=

2

Kt

p

(2)

where K is the Scherrer constant, which was set to 0.9, and is the incident radiation

wavelength (in nm). Since K and p are dimensionless, the unit of the size is the same as the

unit of the wavelength. Further details can be found in ref. 44.

Hence the percent amount of {001} facets, %{001}, was calculated from the surface area of

the {001} and {101} facets, S{001} and S{101}, respectively, according to geometrical

considerations (see Figure 1) using the following formula:

{001}

{001} {101}

%{001} = 100+

S

S S (3)

with

2

{001}= 2 - cot φS w t (4)

7

{101}

2 = 2 - cot φ

sen φ

tS w t (5)

where is the angle between the {001} and {101} facets. Its value was set to 68.3°, which

corresponds to the theoretical angle between the (001) and (101) lattice planes of anatase.

Figure 1. Geometrical model of anatase single crystal: crystal width (w) and thickness (t) are

indicated together with the angle () between the {001} and {101} crystal facets and the

minor base (v) of the trapezoidal {101} facets.

Raman spectra were acquired with a ThermoFisher DXR Raman microscope, using a 532

nm laser (5.0 mW) focused on the sample with a 50× objective (Olympus) to obtain a ca. 1

m spot size. The average values of at least 4 measurements performed on different spots

were considered.

Field-emission scanning electron microscopy (FE-SEM) images were collected by using a

Zeiss SUPRA® 40 scanning electron microscope, operating at a 5-7 kV accelerating voltage,

at a 3-4 mm working distance. Samples, previously sonicated in isopropanol, were dropped on

a silicon wafer and dried prior to measurements.

HR-TEM analyses were carried out with a Zeiss Libra 200 electron microscope operating at

200 kV. Finely ground samples were dispersed in isopropanol, in an ultrasonic bath. A drop

of the suspension was gently deposited on a holey-carbon film supported on a copper grid.

After solvent evaporation, micrographs were taken spanning wide regions of all examined

samples, in order to provide a truly representative statistical map of the powders.

t w

= 68.3°

v

{101}

{001}

8

Nitrogen adsorption/desorption data were collected at 77 K using a Micromeritics Tristar II

3020 V1.03 apparatus, after outgassing at 300 °C for 1 h under a N2 stream. Specific surface

area (SSA) values and pore size distribution were obtained by applying the

Brunauer−Emmett−Teller (BET) and the Barret−Joyner−Halenda (BJH) method, respectively,

to the adsorption isotherms.

X-ray photoemission spectroscopy (XPS) data were collected in a multi-technique ultra-

high-vacuum (UHV) chamber (base pressure: 1.0 × 10−9

mbar) equipped with a VG MKII

ESCALAB electron analyzer (5 channeltrons). Measurements were made at room temperature

in normal emission using a non monochromatized Mg anode X-ray source (hν = 1253.6 eV).

Powder samples were suspended in bi-distilled water and drop-casted on high-purity copper

foils. After drying in air, the obtained films were introduced into the UHV chamber and

outgassed overnight. The charging observed during measurements was corrected by aligning

the Ti 2p3/2 core-level peak signal to 459.0 eV.

Diffuse reflectance (DR) spectra of the photocatalyst powders were recorded on a Jasco V-

670 spectrophotometer equipped with a PIN-757 integrating sphere, using PTFE as a

reference, and then converted into absorption (Abs) spectra (Abs = 1 − R).

2.3. Formic acid photocatalytic oxidation. All formic acid (FA) photocatalytic

degradation runs were performed under atmospheric conditions using the already described

photoreactor and setup,46

including an Osram Powerstar 150 W lamp as irradiation source,

emitting ultraviolet and visible light at > 340 nm, mounted on an optical bench. The light

intensity on the reactor in the 300 - 400 nm range was 5 × 10-8

Einstein s-1

, as calculated from

the emission spectrum of the lamp and its full emission intensity, which was regularly

checked with an optical power meter. The irradiated aqueous suspensions always contained

0.1 g L−1

of photocatalyst and a FA initial concentration equal to 1.0 × 10−3

mol L−1

. The

adsorption equilibrium of the substrate on the photocatalyst surface was attained under

9

magnetic stirring, before starting irradiation. Stirring was continued during the photocatalytic

runs. Portions (3 mL) of the suspension were withdrawn from the photoreactor at different

time intervals during the runs and centrifuged employing an EBA-20 Hettich centrifuge. The

supernatant was analyzed for residual FA content by ion chromatography with conductivity

detection, employing a Metrohm 761 Compact IC instrument, after calibration for formate ion

concentration in the 0−50 ppm range. All kinetic runs were performed up to ca. 70% substrate

degradation, and repeated at least twice, to check their reproducibility.

2.4. Terephthalic acid photocatalytic oxidation. The same photoreactor and setup were

employed to test the efficiency of the photocatalysts in terephthalic acid oxidation, but a

different procedure was followed.47

For each photocatalytic run 10 mg of photocatalyst were

first dispersed in 45 mL of ultrapure water by means of 30-min long sonication. Then a 5 mL

aliquot of a 5 × 10-3

M terephthalic acid solution in 2 × 10-2

M NaOH was added and the

suspension was left under stirring in the dark for 30 min, in order to allow substrate

adsorption on the catalyst surface, prior to irradiation under stirring. Portions of the

suspension collected at fixed time intervals were centrifuged and the concentration of the

fluorescent reaction product was determined in the supernatant. Emission spectra (exc = 315

nm, em = 425 nm) were recorded on an Edinburgh FLS980 spectrofluorimeter, after

calibration.

3. RESULTS AND DISCUSSION

3.1. Phase composition and morphological features. As shown in Figure 2, all samples

match the standard anatase XRPD pattern and no characteristic peaks of other TiO2

polymorphs such as rutile and brookite are observed. In addition, the (100) and (110) peaks of

the cubic TiOF2 phase are clearly observed in the pattern of sample HT_2 at 2 = 23° and 33°,

respectively (see inset (a) in Figure 2). The TiOF2 amount in HT_2 was around 4 wt.%. A

smaller amount of TiOF2 (around 0.3 wt.%) was determined also in sample HT_1 through

10

Rietveld refinement. On the other hand, all samples belonging to the HT_500 series consist of

pure anatase, without any evidence of TiOF2 (see Figure S1).

Figure 2. XRPD patterns of the HT series; inset (a) displays an enlarged portion of the HT_2

sample pattern, showing the presence of the TiOF2 phase; inset (b) shows the overlapped HT

series patterns in the 35° – 50° 2 range, evidencing (103), (004), (112) and (200) anatase

reflections.

20 23 26 29 32 35

I /

a.

u.

2 / degree

(10

0)

(110)

TiOF2

20 30 40 50 60 70 80

I /

a. u

.

2 / degree

(10

1)

(11

6)

(22

0)

(21

5)

(00

4)

(11

2)

(20

0)

(10

5)

(21

1)

(20

4)

HT_0

HT_2

HT_1

HT_0.7

HT_0.5

HT_0.1

(10

3)

35 40 45 50

I /

a.

u.

2 / degree

HT_0

HT_0.1

HT_0.5

HT_0.7

HT_1

HT_2

(004)

(200)

(103)

(112)

(a)

(b)

11

The evolution with the F/Ti ratio of anatase a and c lattice parameters (i.e. the shorter and

the longer lattice parameter in the tetragonal cell of anatase, respectively, see Figure S2 in the

Supporting Information) evidences a clear discontinuity in passing from F/Ti = 0.1 to 0.5: for

lower fluorine contents a increases and c decreases as F/Ti is increased, while for fluorine-

rich materials the reverse is obtained. As for the HT series, the value of c increases up to F/Ti

= 0.7 reaching a sort of plateau for higher F/Ti values (Figure S2b), which is accompanied by

the separation of the TiOF2 phase.

The above findings support progressive insertion of F into the anatase lattice for 0 ≤ F/Ti ≤

0.7, while for F/Ti values larger than 0.7 no more fluorine should be incorporated into

anatase, but it rather segregates into the TiOF2 crystalline phase, that was actually detected in

HT_1 and, especially, HT_2. The formation of TiOF2 may reduce the amount of fluorine in

TiO2, the cell parameter values for HT_2 being between those obtained for HT_0.5 and

HT_0.7. Nevertheless, the crystallite size, especially in the case of crystallites smaller than 10

nm, may also significantly affect the actual value of lattice parameters.48,49

Specific trends of a and c parameters can hardly be identified for calcined samples (HT_500

series in Figure S2). In this case the behavior of c is similar to that of as-prepared samples,

though its relative variation is smaller and no plateau effect is observed for high F/Ti values.

Both effects can be ascribed to the lower residual fluorine amount after annealing.

The increase of fluorine amount has a clear impact on the experimental XRPD patterns of

the HT series in the 35° - 50° 2 range (see inset b of Figure 2). In particular, reflections with

a marked h component (e.g. (200)) shrink with increasing F/Ti ratio, while those with (l > h,

k) show the opposite behavior. This is due to progressive morphological changes which can

be appreciated by comparing the percent amounts of {001} facets reported in Table 1,

calculated according to equation (3).

12

As shown in Figure 3, for the HT series the percentage of exposed {001} facets increases

with increasing the fluorine content up to F/Ti = 1 and then decreases, whereas a sort of

plateau seems to be reached for the HT_500 series at F/Ti ≥ 0.7.

Figure 3. Percent exposed {001} facets obtained from XRPD analysis for the HT (full

squares) and HT_500 (open squares) photocatalysts series as a function of the F/Ti ratio; the

crystal shape expected on the basis of the calculated percentage of {001} facets is shown for

each sample. Estimated standard deviations are covered by data markers.

Calcination at 500 °C causes a general decrease of the {001} facets percent amount (Table

1 and Figure 3), thus determining a smaller degree of truncation, except for HT_0 and

HT_0.1, where no significant variation is observed upon calcination. Therefore annealing

reduces the degree of truncation of anatase crystals, i.e. the v to w ratio (see Figure 1), thus

converting the original plate-like shape of nanocrystals to a more isotropic one.

Besides through XRPD analysis, the percentage of {001} facets was also calculated

according to a recently proposed alternative approach based on Raman spectroscopy,50

as the

intensity ratio of the A1g peak (514 cm-1

) to the Eg peak (144 cm-1

) of anatase. These peaks are

that associated to the antisymmetric bending vibration mode (A1g) of O-Ti-O groups, the

intensity of which increases with the amount of {001} facets, and that arising from the

symmetric stretching vibration modes (Eg), decreasing accordingly. Examples of normalized

0

20

40

60

80

100

0 0.4 0.8 1.2 1.6 2

%{0

01}

nominal F/Ti

HT series

HT_500 series

13

Raman spectra obtained with HT_0.1 and HT_1 are shown in Figure S3. The percent amounts

of {001} facets of our samples calculated by this approach are shown in Figure S4 in

comparison with those obtained by XRPD analysis.

Table 1. Crystal size, percent amount of exposed {001} facets and atomic percent amount

of surface fluorine for the HT and HT_500 photocatalysts series.

sample width (w)

a /

nm

thickness (t)a /

nm % {001}

b % F

c

HT_0 21.7±0.2 31.7±0.9 7.3±0.6 -

HT_0.1 22.3±0.2 25.3±0.6 13.7±0.7 4.0

HT_0.5 41.8±0.4 13.72±0.12 53.4±0.6 3.6

HT_0.7 61.1±1.3 9.88±0.11 72.2±0.9 5.0

HT_1 91±2 8.64±0.09 82.3±0.8 6.5

HT_2 53.2±1.3 10.03±0.15 68.7±1.2 n.a.

HT_500_0 23.6±0.2 31.1±0.6 9.8±0.5 -

HT_500_0.1 22.6±0.2 25.0±0.4 14.3±0.5 1.6

HT_500_0.5 39.5±0.4 19.5±0.2 40.2±0.6 3.0

HT_500_0.7 59.7±0.7 16.9±0.2 57.8±0.7 2.2

HT_500_1 67.1±0.9 18.0±0.2 59.3±0.8 1.8

HT_500_2 59.4±0.8 18.0±0.2 55.8±0.8 n.a.

a Obtained from XRPD measurements.

b Calculated from crystal sizes by assuming a truncated square-bipyramidal shape of anatase

crystals with the angle between (001) and (101) planes fixed at 68.3 °.

c Obtained from the 684.5 eV XPS peak.

14

The results obtained from Raman measurements confirm the trend obtained from XRPD

data for both HT and HT_500 photocatalysts series, with the percent amount of {001} facets

increasing with increasing F/Ti ratio, and are in quite satisfactory agreement for low F/Ti

ratios. However, Raman spectroscopy analysis tends to significantly underestimate the

percent amount of {001} facets in plate-like crystals, at relatively high F/Ti ratios (Figure S4).

By taking into account that the percentage of {001} facets for anatase crystals grown in an

acidic-neutral environment without any specific capping agent is predicted to be around 3-

4%,51

we observe that for HT_0, synthesized under such conditions, the percent amount of

{001} facets determined by the XRPD method is closer to this value than that obtained from

Raman measurements. This points to XRPD as a more reliable method for percent facets

estimation.

Figure 4. HR-TEM images of HT_0, HT_1 and HT_500_1.

15

The morphological effects due to the use of HF as capping agent can be appreciated by

comparing the HR-TEM images obtained for HT_1 with those of reference HT_0. As shown

in Figure 4, HT_1 is incontrovertibly formed by plate-like crystals, the surface of which is

dominated by {001} facets. Their regular shape produces ordered aggregates, in which

crystals are piled up face-to-face, one above the other, thus minimizing the total surface

energy.52

Consequently a large portion of theoretically available {001} surface seems to be

practically inaccessible to substrate adsorption. On the contrary, the casual distribution of

particles displayed by HT_0 aggregates (Figure 4) is coherent with an isotropic shape of

crystals.

On the other hand, the effects induced by the heat treatment here chosen to remove surface

fluorine clearly emerge by comparing the HR-TEM images of HT_1 and HT_500_1 in Figure

4. A plate-like morphology can still be recognized in HT_500_1, though the shape of the

crystals appears quite irregular, leading to a more disordered array, with respect to HT_1

crystals. The resulting structure of the aggregates seems to increase the exposure of {001}

facets, making them more available for substrate adsorption than before calcination.

Furthermore, as shown by both X-ray diffraction data and HR-TEM analysis, the crystal size

markedly increases along the [001] direction upon heat treatment, though slightly decreasing

along the [100] direction, probably due to surface energy minimization, as is the case of

crystal packing into piles observed for HT_1 (Figure 4).

The FE-SEM images of calcined samples shown in Figure 5 evidence an increase in the

side length of crystals with increasing F/Ti ratio, as well as a corresponding decrease in

thickness, in agreement with XRPD findings. Crystal width values span from ca. 20 (for

HT_500_0.1) to ca. 80 nm (for HT_500_1), while thickness ranges from ca. 30 to ca. 7 nm as

the F/Ti ratio increases. Thus the general morphological evolution, from almost isotropic to

plate-like crystals, evidenced by XRPD analysis for the HT series is substantially retained

16

after the thermal treatment. However, after calcination the crystals are more irregular in shape

and this tendency is maximum for HT_500_2, for which the platelet-like geometry can hardly

be recognized (Figure 5). This could be related to the removal of a high amount of fluorine

during the thermal treatment, which also causes the disappearance of TiOF2 evidenced by

phase composition analysis.

Figure 5. FE-SEM images of fluorine-containing samples of the HT_500 series; the

dimension bar corresponds to a 100 nm distance for all images.

3.2. Surface and optical properties. The SSA values obtained for the HT and HT_500

photocatalysts series are reported in Table 2. Samples belonging to the HT series exhibit

similar SSA, possibly increasing with increasing the F content, from 80 m2 g

-1 for F/Ti = 0 to

92 m2 g

-1 for F/Ti = 1.0, while no variation can be appreciated within the HT_500 series, all

SSA values of the 0 F/Ti 1 samples being in the 50 – 59 m2 g

-1 range. As expected,

calcination provokes a general decrease in SSA due to crystallites sintering. Both F/Ti = 2.0

samples in the two series have a SSA markedly lower than those of the other samples in the

same series.

Representative N2 adsorption/desorption isotherms collected for samples belonging to both

series are reported in Figure S5. The presence of a characteristic hysteresis loop allows their

100 nm HT_500_1 100 nm HT_500_2

100 nm HT_500_0.1 100 nm HT_500_0.5 100 nm HT_500_0.7

17

classification as type IV isotherms.53

The observed porosity may originate from the packing

of the TiO2 crystals, since they are not inherently porous.54

With increasing F/Ti ratio, the

hysteresis loops progressively shrink, passing from a H2 type shape, connected to the

presence of the so–called “ink bottle” pores, to a H3 type shape, usually observed with

aggregates of plate-like particles giving rise to slit-shaped pores.

Table 2. Specific surface area of photocatalysts belonging to the HT and HT_500 series.

F/Ti molar

ratio

SSA / m2 g

-1

HT series HT_500 series

0 80.0 ± 0.6 54.9 ± 0.7

0.1 81.4 ± 0.9 59.0 ± 0.8

0.5 84.3 ± 1.0 49.3 ± 0.2

0.7 92.7 ± 0.7 57.2 ± 0.8

1 92.3 ± 0.9 55.5 ± 0.5

2 59.9 ± 1.0 33.8 ± 0.4

Furthermore, for both photocatalysts series the hysteresis loops shift to higher relative

pressures with increasing F/Ti ratio, indicating a progressive pore enlargement. The same

kind of shift of the hysteresis loop was observed for all samples upon calcination, indicating

pore enlargement as well. This can be better appreciated from the shift to larger pore

diameters of the pore size distribution maximum observed after the thermal treatment (see

Figure 6a, showing results obtained with HT_0.7 as a representative sample).

Moreover, the pore size distribution for the HT_500 series reported in Figure 6b shows

similar narrow unimodal distributions which gradually shift toward larger pore diameters with

increasing F/Ti ratio up to 0.7. A markedly different distribution (broader and peaking at

18

larger pore diameters) is obtained for HT_500_1. This indicates a progressive growth in pore

size as the crystals approach the plate-like shape, and also a unique crystal packing for

HT_500_1. Such a peculiarity especially emerges in the comparison with HT_500_0.7,

characterized by a similar percentage of {001} facets. This suggests that the packing observed

after calcination strictly depends on the different shapes of the crystals before annealing.

Thus, an increase of F/Ti ratio promotes the formation of less compact aggregates, containing

larger pores, which become even larger after calcination, in agreement with TEM and FE-

SEM images.

Figure 6. (a) Pore size distribution for samples with F/Ti = 0.7 and 2.0, before and after

calcination; (b) pore size distribution for the HT_500 photocatalysts series.

Interestingly, only HT_2 and HT_500_2 display no hysteresis in isotherm analysis. This

may be attributed to the presence of macropores, consistently with the FE-SEM images of

0

2

4

6

8

10

12

14

0 25 50 75 100 125

dV

/dD

1

03

/ c

m3 g

-1 n

m-1

pore diameter / nm

HT_0.7

HT_500_0.7

HT_2

HT_500_2

(a)

0

2

4

6

8

10

12

0 25 50 75 100 125

dV

/dD

×1

03

/ c

m³

g-1

nm

-1

pore diameter / nm

HT_500_0

HT_500_0.1

HT_500_0.5

HT_500_0.7

HT_500_1

HT_500_2

(b)

19

HT_500_2 (Figure 5) and also with the peculiarly broad pore size distribution of both samples

shown in Figure 6a.

The surface composition of the anatase materials was assessed through XPS analysis. First

of all, almost identical Ti 2p doublet signals, with the two components at binding energy (BE)

464.7 and 459.0 eV, assigned to Ti 2p1/2 and Ti 2p3/2, respectively, were recorded with all

samples. At the same time similar O 1s spectra were obtained with all samples, showing a

main peak centered at 530.2 eV, typical of oxygen in the TiO2 lattice, and a shoulder at higher

BE (531.6 eV), assigned to surface hydroxyl groups.55

Figure 7. XPS spectra in the F 1s region of the HT series samples; the shift of the peak

maximum for HT_2 with respect to the other samples is highlighted.

Concerning the F1s XPS region, the spectra obtained with the HT series are shown in

Figure 7. An asymmetric peak centered at about 684.5 eV, arising from terminal Ti-F bonds,56

was detected for all samples, indicating the presence of surface fluorine. This signal is known

to result from the combination of two different components, which can be resolved by a

deconvolution procedure (see Figure S6).57

The main one, centered at 684.3 eV, is generated

I /

a.

u.

690 688 686 684 682

Binding Energy (eV)

HT_2

HT_1

HT_0.7

HT_0.5

HT_0.1 684.5 eV

685.0 eV

20

by fluorine atoms replacing only one of the two -OH groups coordinated to a surface Ti atom

and the minor one, at 685.3 eV, derives from fluorine atoms coordinated together to the same

Ti center. Notably, the XPS peak maximum for HT_2 is shifted to higher BE with respect to

the other samples, in line with the presence of the crystalline TiOF2 phase 58,59

detected in this

sample.

Another XPS peak at 688.8 eV, usually attributed to substitutional lattice fluorine ions,60

could be identified only in the case of HT_0.1 (see Figure S6) and HT_500_0.1. However the

presence of lattice fluorine also in other samples cannot be ruled out, possibly being below the

detection limit of the XPS technique.57

As shown in Table 1, the amount of surface fluorine detected by XPS analysis slightly

increases with increasing F/Ti ratio in the HT series, while a significantly lower, quite

uniform F-coverage was detected in the HT_500 series (except for HT_500_0.5).

The DR spectra of the photocatalysts (see Figure S7) show the usual absorption onset of

anatase at < 400 nm, with samples synthesized in the presence of the fluorine capping agent

possibly exhibiting a slightly blue-shifted absorption edge. A similar,61

as well as the opposite

behavior18,22,47

have been reported in the literature with an open debate about the dependence

of the anatase band gap on facet composition. The different crystal size, which significantly

affects the band gap, could be responsible for the observed discrepancies, even more than the

percentage of exposed {001} facets.39

Anyway, recent theoretical and experimental work26,62

indicate that the band gap should not depend on the amount of exposed specific facets.

Finally, the heat treatment at 500 °C eliminates the low, almost uniform absorption

contribution extending over the whole visible range, possibly originated from residual surface

carbon after the synthesis (see HT_1 vs. HT_500_1 spectra in Figures8.

3.3. Photocatalytic oxidation of formic acid. The photocatalytic degradation of FA

occurred at constant rate, i.e. according to a zeroth-order rate law, as in previous studies.32,63,64

21

Therefore the activity of the here investigated TiO2 materials in FA photocatalytic oxidation

can be compared in terms of zeroth-order rate constants k, which were normalized for the

photocatalysts’ surface area, to better extract their intrinsic photoactivity. Surface normalized

rate constants of FA photocatalytic oxidation obtained employing the photocatalysts with a

nominal F/Ti ratio ranging from 0 to 1 are reported in Figure 8, together with the residual

surface fluorine percent amount detected on their surface by XPS analysis.

Figure 8. Surface normalized rate constants of FA photomineralization (filled columns in the

upper panel) and percent surface fluorine amount detected by XPS analysis (patterned

columns in the lower panel, in reverse axis scale) along with percentages of {001} facets

obtained from XRPD analysis (circles, upper panel) for both HT and HT_500 series.

First of all, the photoactivity of fluorine-containing samples of the HT series are relatively

low and do not exhibit any trend with the nominal F/Ti ratio, as already reported for as-

prepared fluorine-containing photocatalysts.18

Reference HT_0 clearly displayed superior

photocatalytic activity among HT series photocatalysts. After heat treatment at 500 °C all

photocatalysts show higher photoactivity. In particular, HT_500_1 is by far the best

photocatalyst, exhibiting an activity similar to that of benchmark P25 TiO2 from Evonik [k =

(4.76 0.15) 10-7

M s-1

m-2

] and better than that of reference sample HT_500_0. Moreover,

2.4

1.00.7

1.01.1

3.7

1.4 1.3

2.1

4.9

0

20

40

60

80

100

0

1

2

3

4

5

6

%{0

01

}

k ×

10

7/ M

s-1

m-2

k - HT k - HT_500

%{001} - HT %{001} - HT_500

4.03.6

5.0

6.5

1.6

3.0

2.21.8

0

1

2

3

4

5

6

7

0 0.1 0.5 0.7 1

F a

t.%

nominal F/Ti

F% - HT F% - HT_500

2.4

1.00.7

1.01.1

3.7

1.4 1.3

2.1

4.9

0

20

40

60

80

100

0

1

2

3

4

5

6

%{0

01

}

k ×

10

7/ M

s-1

m-2

k - HT k - HT_500

%{001} - HT %{001} - HT_500

22

an overall photoactivity increase can be clearly appreciated with increasing the {001} facet

percentage for F-containing samples (Figure 8).

The general improvement in photoactivity observed upon calcination can be ascribed to

different effects brought about by the thermal treatment. Firstly, the partial removal of surface

fluorine (together with other adsorbed species), proved by XPS analysis, certainly contributes

in increasing the photoactivity in this specific test reaction. In fact residual fluorine is

expected to inhibit the adsorption of FA, thus preventing its degradation, which occurs

through direct oxidation on the TiO2 surface.32

Moreover, the F-terminated surface, displaying

a lower amount of surface –OH groups, is less efficient in creating ≡Ti-O• effective traps for

photogenerated valence band holes, according to the following reaction:65

≡Ti-OH + hVB+ → ≡Ti-O

• + H

+ (6)

but it rather produces free •OH radicals as oxidizing species:

≡Ti-F + hVB+ + H2O → ≡Ti-F +

•OH + H

+ (7)

Calcination is also expected to increase the photocatalytic activity as a consequence of the

increased crystallinity of the material and also of the formation of beneficial surface trapping

sites for charge carriers,66

when fluorine is removed from the titania surface.

All these factors contribute to the general photoactivity improvement displayed by all

photocatalysts upon calcination, irrespective of their crystal morphology. However, by

carefully considering how photoactivity varies within the HT_500 series, a clear contribution

of crystal shape can be recognized, regardless of fluorine removal. In fact, for similar amounts

of residual surface fluorine, the photocatalytic activity of plate-like HT_500_1 is far higher

than that of the slightly truncated HT_500_0.1. Furthermore, HT_500_0.5 and HT_500_0.7,

featuring a considerable percentage of {001} facets, show comparable and better

performances, respectively, than HT_500_0.1, though bearing larger amounts of fluorine on

their surface (Table 1).

23

Thus, the superior activity of the {001} facets is fully revealed only upon calcination,

inducing not only surface fluorine removal, but also favorable changes in crystal morphology.

In fact, as-prepared crystals with large F/Ti ratios are regularly-shaped nanosheets, piled in an

ordered face-to-face fashion (Figure 4). Under such conditions, a significant percentage of

{001} facets is not available to FA adsorption. On the contrary, a beneficial rearrangement in

the crystal packing is favored by their irregular shape after calcination at 500 °C, though they

still retain the desired platelet-like morphology, as clearly shown by electron microscopy

images (Figure 4 and 5) as well as by XRPD analysis (Table 1). Therefore, even if the degree

of truncation of TiO2 nanocrystals decreases upon calcination, the morphology changes may

result in an increase of the actually exposed {001} surface, allowing for adsorption (and thus

faster degradation) of FA.

Such changes are reflected in the porosity of the materials after calcination. Its relevant

contribution to photoactivity has been fully evidenced here for the first time and appears to

depend on the original morphology of the crystals. In fact, among calcined samples,

HT_500_1 seems to particularly benefit from a favorable crystal aggregation, since its

photoactivity markedly differs from that of HT_500_0.7, though the two samples bear similar

percent amounts of {001} facets. This finding is supported by the unique porosity profile

characterizing only HT_500_1 among all HT_500 series samples (Figure 6b) that, along with

the highest percentage of {001} facets and lowest amount of surface fluorine, makes it the

best performing photocatalyst.

The improved photocatalytic activity might not be simply ascribed to a larger amount of

{001} facets, but may result from an appropriate interplay between different facets.10,11

Indeed, it has been reported that {001} and {101} facets differ in the location of their band

edges, so that electrons preferably migrate to {101} facets, while holes tend to move to {001}

facets.26,35

Thus, the coexistence of such facets in a proper balance allows spatial separation of

24

photogenerated charges and thus higher photoactivity. Notably, the percent amount of {001}

facets in most efficient HT_500_1 in FA degradation (59%) is very similar to that recently

found to be optimal to hinder the undesired electron-hole recombination (55%).26

3.4. Photocatalytic oxidation of terephthalic acid. The conversion of terephthalic acid

(TA) into 2-hydroxyterephthalic acid (TAOH) was employed to investigate how anatase

morphology in better performing HT_500 photocatalysts with a similar surface residual

fluorine amount (1.8 ± 0.2 at.%) influences the photocatalytic production of hydroxyl radicals

from water molecules oxidation. The so produced hydroxyl radicals then attack organic

substrates through the so called indirect oxidation mechanism.

Figure 9. Concentration of TAOH per photocatalyst unit surface area after 45 min irradiation

(filled columns in the upper panel) and percent surface fluorine amount detected by XPS

analysis (patterned columns in the lower panel, in reverse axis scale) along with dispersion

plots (upper panel) of the percentage of {001} facets obtained from XRPD for selected

samples of the HT_500 series.

The amount of photogenerated •OH radicals, trapped in the form of fluorescent TAOH, was

found to increase during irradiation of the photocatalyst suspensions, as shown in Figure S8.

TAOH concentrations detected in the irradiated suspension after 45 min-long irradiation,

1.9

1.4

2.1

2.9

0

15

30

45

60

75

0

1

2

3

%{0

01

}

[TA

OH

] ×

10

5/ M

m-2

[TAOH] after 45'

%{001}

1.6

2.2

1.8

0

1

2

HT_500_0 HT_500_0.1 HT_500_0.7 HT_500_1

F a

t.%

F at.%

25

normalized with respect to the surface area of each photocatalyst, are compared in Figure 9

together with the residual percent amount of surface fluorine species detected by XPS

analysis in HT_500 samples. The photoactivity scale observed for FA mineralization was

found to be substantially retained in this reaction too. Also HT_500_0.7, besides HT_500_1,

proved to be better than fluorine-free HT_500_0.

Thus, the photoefficiency in •OH radicals generation is also favored by {001} facets

exposure. In this case the influence of residual capping fluorine on both substrate adsorption

and •OH production (according to equation (7)) should play a minor role, this test reaction

proceeding in basic medium, i.e. with surface fluorine being displaced by terminal −OH

groups.67,68

We thus demonstrate that the rate of both direct and •OH-mediated photocatalytic oxidation

paths is enhanced for anatase with a plate-like morphology, pointing to a superior oxidation

ability of {001} facets either alone, or in combination with the most thermodynamically stable

{101} facets.

4. CONCLUSIONS

We demonstrate here that the superior photoactivity of {001} facet-enriched anatase TiO2 is

revealed only upon calcination, which not only removes excess residual fluorine that may

hinder substrate adsorption, but also triggers morphological changes in crystal aggregates,

with a consequent exposure of a larger fraction of {001} facets. Ultimately, the here chosen

annealing conditions increase the portion of {001} surface facets effectively available for

photocatalytic processes, at the same time essentially preserving the desired plate-like crystal

anisotropy. Both direct and •OH-mediated photocatalytic oxidation paths are promoted by

larger amounts of exposed {001} facets, also possibly coexisting with {101} facets in an

appropriate proportion. The best photoactivity in both test reactions was attained by a

photocatalyst featuring about 60% of {001} facets.

26

ASSOCIATED CONTENT

Supporting information. Amounts of employed reagents, preliminary photoactivity tests,

XRPD pattern of calcined samples, anatase lattice parameters, Raman spectra, percent

amounts of exposed {001} facets according to XRPD and Raman spectroscopy approaches,

nitrogen adsorption isotherms, deconvolution of XPS spectra in the F 1s region, UV-vis DR

spectra, sample fluorescence spectra of photoproduced TAOH. This material is available free

of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding authors

*M. V. Dozzi. E-mail: mariavittoria.dozzi@unimi.it.

*E. Selli. E-mail: elena.selli@unimi.it.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS

The collaboration of Dr. Mariangela Longhi in collecting nitrogen adsorption isotherms and

of Alessio Zuliani in artwork creation are gratefully acknowledged. The use of the

instrumentation purchased through the Regione Lombardia – Fondazione Cariplo joint

SmartMatLab project (Fondazione Cariplo grant 2013-1766) is also acknowledged. This work

received support from the Fondazione Cariplo project Novel Photocatalytic Materials Based

on Heterojunctions for Solar Energy Conversion (grant 2013-0615).

27

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doi:10.1016/j.apcata.2015.10.048

ABSTRACT GRAPHIC

SPECIFIC SURFACE

AREA

{001} FACETS

AMOUNT

SURFACE FLUORINE

CRYSTAL AGGREGATES

001-FACET

ENRICHED ANATASE

PHOTOACTIVITY